The only problem I see with Maokai

Is that the whole thing is called the tank update yet Maokai was stripped of any synergy for defenses on his kit :S without damage reduction why would you build tank?

I think Maokai support could theoretically work though because he's all about utility now, I mean that ult besides an ADC can be crazy good but in top lane you'll get decimated by champions having more damage AND more defenses than you naturally...

Sejuani gained a defensive buff that gives her almost 100% dmg reduction, it's crazy good for a tank and ZAC has always had his blobs plus his revive passive, being tanky makes sense because you technically have 2 health bars and now you need to survive long enough for a huge reward (potentially displacing the enemy team)

But Maokai only has his heal scaling off of max HP now (and you took HP from items...) nothing is stopping people from buying AP or even support items and at this point they look WAY more attractive than tank, Sapplings doing +2%hp per 100 AP is crazy good and his ult having a 75% AP ratio is nuts

TL;DR: On the tank update, you erased one tank ._. wat

Buuuuuut Maokai became a drain tank now D: a very different one from those already in the game though and that's simply incredible, check pants are dragon video here: https://www.youtube.com/watch?v=FuCAnPuOpMQ and see how awesome Maokai can be when played accordingly to his strengths (even though pants wasn't particularly playing super focused, he knew that Mao had changed quite a bit and tried to addapt to his new kit)